The present invention relates generally to the detection and measurement of forces and more particularly to an improved accelerometer incorporating one or more vibrating force transducers for measuring the force applied to a proof mass. The present invention also relates to a method for manufacturing the accelerometer.
A widely used technique for force detection and measurement employs a mechanical resonator having a frequency of vibration proportional to the force applied. In one such mechanical resonator, one or more elongate beams are coupled between an instrument frame and a proof mass suspended by a flexure. A force, which may be electrostatic, electromagnetic or piezoelectric, is applied to the beams to cause them to vibrate transversely at a resonant frequency. The mechanical resonator is designed so that force applied to the proof mass along a fixed axis will cause tension or compression of the beams, which varies the frequency of the vibrating beams. The force applied to the proof mass is quantified by measuring the change in vibration frequency of the beams.
Recently, vibratory force transducers have been fabricated from a body of semiconductor material, such as silicon, by micromachining techniques. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove unmasked portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical resonator device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. Nos. 5,006,487, xe2x80x9cMethod of Making an Electrostatic Silicon Accelerometerxe2x80x9d and 4,945,765 xe2x80x9cSilicon Micromachined Accelerometerxe2x80x9d, the complete disclosures of which are incorporated herein by reference.
The present invention is particularly concerned with Accelerometers having vibrating beams driven by electrostatic forces. In one method of fabricating such miniature accelerometers, a thin layer of silicon, on the order of about 20 micrometers thick, is epitaxially grown on a planar surface of a silicon substrate. The epitaxial layer is etched, preferably by reactive ion etching in a suitable plasma, to form the vibrating components of one or more vibratory force transducers (i.e., vibrating beams and electrodes). The opposite surface of the substrate is etched to form a proof mass suspended from a stationary frame by one or more flexure hinge(s). While the opposite surface of the substrate is being etched, the epitaxial layer is typically held at an electric potential to prevent undesirable etching of the epitaxial layer. During operation of the transducer, the beams and the electrodes are electrically isolated from the substrate by back biasing a diode junction between the epitaxial layer and the substrate. The transducer may then be coupled to a suitable electrical circuit to provide the electrical signals required for operation. In silicon vibrating beam accelerometers, for example, the beams are capacitively coupled to the electrodes, and then both the beams and electrodes are connected to an oscillator circuit.
The above described method of manufacturing force detection devices suffers from a number of drawbacks. One such drawback is that the beams and electrodes of the vibratory force transducer(s) are often not sufficiently electrically isolated from the underlying substrate. At high operating temperatures, for example, electric charge or current may leak across the diode junction between the substrate and the epitaxial layer, thereby degrading the performance of the transducer(s). Another drawback with this method is that it is difficult to etch the substrate without etching the epitaxial layer (even when the epitaxial layer is held at an electric potential). This undesirable etching of the epitaxial layer may reduce the accuracy of the transducer.
Another drawback with many existing force detection devices, such as accelerometers, is that they often have an asymmetrical design, which may make it more difficult to incorporate the accelerometer into a system, particularly in high performance applications. For example, the proof mass flexure hinge is typically etched on the opposite surface of the substrate as the transducers. This produces an asymmetrical device in which the input axis of the accelerometer is skewed relative to a direction normal to the surface of the silicon wafer.
Pendulous accelerometers, for example, vibrating beam accelerometers, capacitive accelerometers, capacitive rebalance accelerometers, and translational mass accelerometers comprise a reaction mass. Existing design and manufacturing techniques for these devices are described in U.S. Pat. Nos. 4,495,815 xe2x80x9cMass And Coil Arrangement For Use In An Accelerometer,xe2x80x9d 5,396,798 xe2x80x9cMechanical Resonance, Silicon Accelerometer,xe2x80x9d 4,766,768 xe2x80x9cAccelerometer With Isolator For Common Mode Inputs,xe2x80x9d 5,228,341 xe2x80x9cCapacitive Acceleration Detector Having Reduced Mass Portion,xe2x80x9d 5,350,189 xe2x80x9cCapacitance Type Accelerometer For Air Bag System,xe2x80x9d 4,335,611 xe2x80x9cAccelerometer,xe2x80x9d and 3,702,073 xe2x80x9cAccelerometerxe2x80x9d which are incorporated herein by reference. All practical pendulous accelerometers to date function on the principle of Neuton""s law that force equals mass times acceleration. In many accelerometer applications high performance and small size are desirable. One problem with the design of small, high performance pendulous accelerometer sensors involves obtaining adequate reaction mass in a small space. A second problem with the design of small, high performance pendulous accelerometer sensors involves providing adequate isolation from the mounting structure such that mounting strains do not affect accelerometer performance. Typical accelerometer sensors include a pendulous reaction mass, often referred to as a proof mass, suspended from a stationary frame by, for example, a flexural suspension member of some other form of pivot mechanism. This pivot constrains the reaction mass to only one direction of motion; the reaction mass is free to move along this one direction of motion unless restrained to the null position. The pendulous reaction mass must be restrained under acceleration by an opposing force which may be the result of a position feedback circuit. Alternatively, the accelerometer may be an open-loop device in which the opposing force may be supplied a spring in the form of, for example, pivot stiffness. In a typical accelerometer sensor mechanism the pendulous reaction mass is suspended on a flexural suspension member inside an external support frame. Isolation is typically provided by mounting the supporting frame itself inside an isolation feature supported from a final exterior frame which provides mounting both to sensor covers and to the accelerometer housing. The above features as practiced in a typical vibrating beam accelerometer sensor are shown in FIGS. 1 and 2. The large exterior frame system is static and adds no mass to the active reaction mass. Additionally, any external strain couples through the exterior frame system directly across the length of the sensor mechanism. The resulting large frame dimensions tend to maximize the effect of error drivers, for example, thermal expansion mismatch, placing additional burden on the isolator function.
The present invention provides methods for detecting and measuring forces with mechanical resonators and improved methods of manufacturing these force detecting apparatus. These methods and apparatus are useful in a variety of applications, and they are particularly useful for measuring acceleration.
The present invention includes a substrate coupled to a thin active layer each comprising a semiconducting material. The substrate has a frame and a proof mass suspended from the frame by one or more flexures. The active layer includes one or more vibratory force transducers suitably coupled to the proof mass for detecting a force applied to the proof mass. According to the present invention, an insulating layer is formed between the substrate and the active layer to insulate the active layer from the substrate. Providing a separate insulating layer between the substrate and active layer improves the electrical insulation between the proof mass and the transducers, which allows for effective transducer operation over a wide range of temperatures.
In a specific configuration, the substrate and active layer are made from a silicon material, ane the insulating layer comprises a thin layer (e.g., about 0.1 to 10.0 macrometers) of oxide, such as silicon oxide. The silicon oxide layer retains its insulating properties over a wide temperature range to ensure effective transducer performance at, for example, high operating temperatures on the order of above about 70xc2x0 C. to 100xc2x0 C. In addition, the insulating layer inhibits undesirable etching of the active layer while the substrate is being etched, which improves the accuracy of the apparatus.
In a preferred configuration, the flexure hinge of the proof mass is preferably etched near or at the center of the silicon substrate that comprises the proof mass (i.e., substantially centered between the first and second surfaces of the substrate). This arrangement provides an input axis that is substantially normal to the surface of the substrate, thereby improving the alignment.
In an exemplary embodiment, the force detecting apparatus comprises an accelerometer for measuring the acceleration of the stationary frame relative to the proof mass. In this embodiment, the active layer includes a pair of vibratory force transducers on either side of the proof mass. The vibratory force transducers each preferably include first and second parallel beams each having a first end portion fixed to the proof mass, a second end portion fixed to the instrument frame and a resonating portion therebetween. The transducers each further include first and second electrodes positioned adjacent to and laterally spaced from the first and second beams. An oscillating circuit is capacitively coupled to the electrodes for electrostatically vibrating the beams and for determining a magnitude of a force applied to the proof mass based on the vibration frequency of the beams.
The accelerometer of the present invention is manufactured by applying an insulating layer of silicon oxide between the silicon substrate and the active layer. Preferably, the silicon oxide layer is first deposited or grown onto substantially planar surfaces of the substrate and the active layer, and then the substrate and active layer are bonded together, e.g., with high temperatures, so that the silicon oxide layers insulate the substrate from the active layer. In a preferred configuration, portions of the silicon wafers will be removed after they have been bonded together to provide a substrate of about 300 to 700 micrometers and a relatively thin active layer of about 5 to 40 micrometers bonded thereto. The proof mass and instrument frame are then etched into the substrate and the transducers are etched, preferably with reactive ion etching, into the active layer. The insulating layer inhibits undesirable etching of the active layer while the substrate is being etched and vice versa. Forming the accelerometer components from the silicon wafers results in the transducer beams being mechanically coupled to the proof mass and the frame. Both the beams and the electrodes are then coupled to a suitable external oscillator circuit.
Additionally, the present invention resolves significant problems of the prior art by providing both superior mounting stress isolation and substantially reduced acceleration sensor mechanism size while maintaining adequate mass in the reaction mass without increasing manufacturing costs. In the present invention the external frame isolation system is eliminated and the remaining structure becomes the active reaction mass. The present invention describes various embodiments optimized for various g-range applications. The illustrated embodiments substantially reduce mechanism size and maximize active mass while maximizing isolation from external error sources and minimizing heat flow.